Tiles are used to build walls, floors, and ceilings. Along with doors and airlocks, they enclose rooms.
Basic tiles can be constructed from any mineral and provide Duplicant run speed bonuses along with decor, based on the mineral used. They block airflow. Liquids will spread across horizontal tiles (and small amounts can be cleaned by mopping), and vertical tiles will block liquids.
Tile is also used as a basic unit of distance, e.g. Duplicants are 2 tiles tall.
Besides simple walls and floors, tiles can create U-shaped pools for holding water, insulation against hot zones, and cages over sleeping hatches.